6.7 Ground Data

6.7 Ground Data

Ground data, in some cases called ground "truth" is defined as the observation, measurement and collection of information about the actual conditions on the ground in order to determine the relationship between remote sensing data and the object to be observed. Investigation on the sea is sometimes called sea truth. Generally ground data should be collected at the same time as data acquisition by the remote sensor, or at least within the time that the environmental condition does not change. It should not be inferred that the use of the word "truth" implies that ground truth data is not without error. Ground data is used as for sensor design, calibration and validation, and supplemental use, as shown in Figure 6.7.1.

For the sensor design, spectral characteristics are measured by a spectrometer to determine the optimum wavelength range and the band width.

For supplemental purposes, there are two applications; analysis and data correction. The former case, for example, is ground investigation, at a test area, to collect training sample data for classification. The latter case, for example, is a survey of ground control points for geometric correction.

The items to be investigated by ground data are as follows.

a. Information about the object type, status, spectral characteristics, circumstances, surface temperature etc.

b. Information about the environment, the sun azimuth and elevation, irradiance of the sun, atmospheric clarity, air temperature, humidity, wind direction, wind velocity, ground surface condition, dew, precipitation, etc.

Depending on the purpose, the above items and the time of ground investigation should be carefully selected.

Ground data will mainly include identification of the object to be observed, and measurement by a spectrometer, as well as visual interpretation of aerial photographs and survey by existing maps, and a review of existing literature and statistics.

Figure 6.7.2 shows data collection from various altitudes including ground data.

As the collection of ground data is time consuming as well as expensive, it is best to establish a test site for sensor design, calibration and validation, and data correction. The test area should be carefully selected with respect to ease of survey, variety of features present, weather condition and so on.